RSC Advances

View Article Online PAPER View Journal | View Issue

Synthesis of carbon nano-onion and nickel hydroxide/ oxide composites as supercapacitor electrodes Cite this: RSC Adv., 2013, 3, 25891 Marta E. Plonska-Brzezinska,*a Diana M. Brus,a Agust´ın Molina-Ontoriab and Luis Echegoyen*b

“Small” carbon nano-onions (CNOs) are spherical, ca. 5 nm in diameter, concentric shells of graphitic carbon that can be also described as multi-shelled fullerenes. Given the easy functionalization and high thermal stability of the CNOs produced from nanodiamond, they are the most obvious choice for studying the potential applications of these multi-shelled fullerenes in electrochemical supercapacitors (ES). Since limited accessibility of the carbon surface to electrolyte penetration is observed for carbon nano-onions, performance enhancement was accomplished by modifying the CNO surfaces with pseudocapacitive

redox materials: Ni(OH)2 and NiO. These composites were characterized by TEM, SEM, XRD, TGA-DTG- Received 8th August 2013 DTA and Raman spectroscopy. The electrochemical properties of these composites were also Accepted 10th October 2013 investigated. Compared with pristine CNOs (30.6 F g1 at 5 mV s1), modiﬁed CNOs (1225.2 F g1 for DOI: 10.1039/c3ra44249g 1 1 CNOs/Ni(OH)2 and 290.6 F g for CNOs/NiO, both at 5 mV s ) show improved electrochemical www.rsc.org/advances performance, promising for the development of supercapacitors.

1. Introduction Many systems have been developed as electrode materials for supercapacitors. Pseudocapacitors are generally classied into An electrochemical capacitor, also called an electrochemical two types: conductive polymers6 and electroactive transition- supercapacitor (ES) or ultracapacitor, is a device able to store metal oxides/hydroxides.7 Conductive polymers have been charge at the electrode–electrolyte interface.1 This accumula- widely investigated for fabricating supercapacitors, because tion of charge is mainly based on two types of processes: elec- they have multiple redox states, good environmental stability, trostatic interactions (electrical double layer capacitors, EDLC),2 and they are inexpensively and easily fabricated into various or faradaic processes (pseudocapacitors, also called faradaic nanostructures.8 However, they usually suﬀer from degradation supercapacitors, FS).3 In EDLCs the energy can be stored via ion resulting from the repeated charge–discharge processes,

Published on 15 October 2013. Downloaded by The University of Texas at El Paso (UTEP) 05/05/2015 21:37:28. adsorption and the capacitance is associated with an electrode- leading to poor cycling performance. To improve the specic potential-dependent accumulation of charge at the electrode capacitance and the energy density, transition metal oxides/ interface through polarization. During the charging–discharg- hydroxides are being investigated as alternative materials for ing process of carbon-based EDLCs, the electrode material is supercapacitor electrodes. Generally, using metal oxides/ not electrochemically active. In pseudocapacitors the electrode hydroxides in ES requires: (i) electronic conductivity, (ii) two or material is electrochemically active and faradaic reactions take more oxidation states that coexist over a continuous potential place, which are also involved in the charge storage/delivery range with no phase changes and (iii) protons to intercalate into process. The type of charge storage mechanism determines the the oxide lattice upon reduction (and out of the lattice upon electrochemical performance of ECs and the electrode materials oxidation).9 To date, the systems investigated include oxides/ are also crucial. Ideal electrode materials are required to hydroxides of ruthenium,10 manganese,11 cobalt,12 nickel,13 and possess: a high specic surface area, controlled porosity, high vanadium.14 These materials exhibit reasonable pseudocapaci- charge exchange rates, high electronic conductivity, good power tive performance. On the other hand, metal oxides/hydroxides density, high thermal and chemical stability, and preferably low may not be employed alone as supercapacitor electrodes due to: costs of the raw materials and manufacturing.4,5 (i) very low conductivity, (ii) poor power density, (iii) low charge exchange rates, (iv) poor long-term stability during the charge– discharge processes that lead to cracking of the electrode, and (v) low surface area and non-homogeneous pore distributions. a Institute of Chemistry, University of Bialystok, Hurtowa 1, 15-399 Bialystok, Poland. Nickel oxide/hydroxide is one of the most widely used E-mail: [email protected]; Fax: +48 85 747 0113; Tel: +48 85 745 7816 materials in pseudocapacitors. It has been determined that bDepartment of Chemistry, University of Texas at El Paso, 500 W. University Ave., El  Paso, TX 79968, USA. E-mail: [email protected]; Fax: +1 915 747 6801; Tel: +1 Ni(OH)2 can yield much higher speci c capacitances than 915 747 7573 NiO.15 Nanostructures with diﬀerent morphologies like wires,

This journal is ª The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 25891–25901 | 25891 View Article Online RSC Advances Paper

urchins, cones, balls with ripple-shaped pores, and belts have of nickel hydroxide and oxide is associated with rapid proton 16  ﬀ 36 been fabricated and tested. The speci c capacitance of the di usion in the Ni(OH)2 and NiO frameworks. In order to nickel hydroxides depend on the structures, but all exhibit obtain a high charge exchange rate, it is necessary to prepare values that are far from their theoretical maximum, 2573 F g 1 nanosized inorganic materials. Recently, the design of a new at 0.5 V,17 depending on the synthesis method and the type of NiO/C nanocapsule with onion-like C shells was pub- morphology.18 Therefore, it is important to investigate how to lished which exhibited a high charge–discharge rate and an maximize the electrochemical performance of nickel hydroxide. excellent cycling stability.37 1 For instance, a high specic capacitance of 1478 F g with In the present work, CNO/Ni(OH)2 composites were prepared

excellent charge exchange rates and long cycle ability was by loading of Ni(OH)2 on the carbon structures, followed by obtained for a nickel hydroxide lm calcinated at 250 C.19 The calcination to obtain CNO/NiO. The composites were charac- typical specic capacitance based on nickel hydroxide elec- terized by TEM, SEM, XRD, TGA-DTG-DTA, FT-IR and Raman trodes ranges from 50 to 1776 F g 1.20 To our knowledge, the spectroscopy. The electrochemical properties of the hybrid highest specic capacitance, 3152 F g 1, was achieved for systems in an aqueous electrolyte were also examined.

electrodeposited Ni(OH)2 on nickel foam in a 3% KOH solution – 1 21 at a charge discharge current density of 4 A g . 2. Experimental section The electrochemical performance of pseudocapacitors depends mainly on the synthetic procedures used. Several 2.1. Materials

synthetic strategies can be exploited to fabricate mixed- The following chemicals were used as received: nickel(II) nitrate composites containing metal oxides including hydrothermal hexahydrate (>97.0%, Sigma-Aldrich), ethyl alcohol absolute 22 – 23 24 synthesis, sol gel chemistry, electrodeposition or spray (99.8%, POCH), ammonia hydrate (25%, POCH), conductive 25 pyrolysis. The method allows to control electrode micro- carbon paint – a dispersion of colloidal graphite (20% solids) in structures, morphology and homogeneity. Recently, work with isopropyl alcohol, polyvinylpyrrolidone (PVP) ($99.0%, Sigma- supercapacitors has been focused on designing and synthe- Aldrich), (4-dimethylamino)pyridine (4-DMAP) ($99.0%, sizing composite electrode materials to fully exploit their Sigma-Aldrich), pyridinium p-toluenesulfonate polymer-bound advantages and overcome their disadvantages. Some reports (PPST) (Sigma-Aldrich), nanodiamond powder (Carbodeon  showed that high speci c capacitances can be obtained if metal uDiamond Molto) with a crystal size between 4 and 6 nm, and oxides or hydroxides are uniformly dispersed on carbon mate- nanodiamond content $97 wt%. 26 rials with high surface areas. Combining metal oxide/ Commercially available nanodiamond powder with a crystal  hydroxide with carbon nanomaterials could: enhance speci c size between 4 and 6 nm was used for the preparation of CNOs. surface areas, induce high porosity, facilitate electron and NDs were placed in a graphite crucible and transferred to an proton conduction, expand active sites, extend the potential Astro carbonization furnace. The air in the furnace was removed window, protect active materials from mechanical degradation, by applying a vacuum followed by purging with helium. The improve cycling stability and provide extra pseudocapacitance. process was repeated twice to ensure complete removal of air. Various carbon-based composites are currently being investi- Annealing of ultradispersed nanodiamonds was performed at gated as supercapacitor electrodes because of the synergistic 1650 C under a 1.1 MPa He atmosphere with a heating ramp of properties arising from the carbon materials (high power 20 Cmin 1.15 The nal temperature was maintained for one Published on 15 October 2013. Downloaded by The University of Texas at El Paso (UTEP) 05/05/2015 21:37:28. density) and from the pseudo-capacitive nanomaterials (high hour, then the material was slowly cooled to room temperature 27 28 energy density), including nickel oxides/hydroxides. over a period of one hour. The furnace was opened, and the CNOs New types of supercapacitors which combine the advantages were annealed in air at 400 C to remove any amorphous carbon. of both double layer capacitors and pseudocapacitors are the All aqueous solutions for electrochemical studies were subject of this work. To the best of our knowledge, this is the prepared using deionized water of 18.2 MU resistivity, which  “ ” rst composite synthesis of small carbon nano-onions (CNOs) was further puried with a Milli-Q system (Millipore). with nickel oxides/hydroxides. The CNO structures consist of a hollow spherical fullerene core surrounded by concentric and curved graphene layers with increasing diameters. The high 2.2. Synthesis of nickel hydroxide temperature annealing of ultra-dispersed nanodiamonds 5 g of nickel nitrate hexahydrate was dissolved in 40 mL of water (5 nm, average size) leads to their transformation into CNO and ethanol in a volume 1 : 1 ratio and stirred. To this solution structures (5–6 nm in diameter, 6–8 shells).29 The interest in was added 5% ammonia hydrate dropwise to achieve a pH ¼ carbon nano-onions is driven by their unusual physico-chem- 9.5. The precipitate was centrifuged and washed several times ical properties as well as by promising applications in elec- with ethanol to remove ammonia hydrate. The resulting aqua- tronics,30,31 optics,32 biosensors,33 as hyperlubricants,34 and in marine solid was dried overnight in an oven at 60 C. energy conversion and storage.35 50 mg of the selected substance, PVP, 4-DMAP or PPST, was The number of published articles describing CNO composite dissolved in a mixture (40 mL) of water and anhydrous ethanol preparation is still very sparse. Non-modied CNOs have in a 1 : 1 ratio and sonicated for 15 minutes. Subsequently, 5 g limited charge accumulation properties. Combining the carbon of nickel nitrate hexahydrate was dispersed in this solution and material with an inorganic one should lead to electrochemical stirred. During stirring, 5% ammonia hydrate solution was properties from both of them. The fast charging rate capability added dropwise until the pH ¼ 9.5. The suspension was

25892 | RSC Adv., 2013, 3, 25891–25901 This journal is ª The Royal Society of Chemistry 2013 View Article Online Paper RSC Advances

centrifuged. The precipitate was collected and washed several with alumina powder on polishing cloth. The electrode was times with ethanol. Finally, the crude product was dried over- immersed in ethanol, sonicated for a few minutes and washed night in an oven at 60 C. with water to remove all impurities. The lm on the GC electrode was prepared as follows: 2 mg of active material was dis-

2.3. Preparation of composites of CNOs/4-DMAP/Ni(OH)2 solved in a solution containing conductive carbon paint and ethanol in a volume ratio of 1 : 6 and sonicated for 15 minutes. 10 mg of CNOs was dispersed in 3 mL of anhydrous ethanol and Subsequently, the surface of the working electrode was modi- mixed with 3 mL of an aqueous solution of (4-dimethylamino) ed by the drop-coating method. A saturated calomel electrode pyridine (5 mg). This suspension was placed in an ultrasonic (SCE) was used as the reference electrode and a Pt wire served as bath for 15 minutes. Nickel nitrate hexahydrate (500 mg) was the counter electrode. The electrochemical measurements were then added and the mixture was stirred. A 5% ammonium performed in 1 M KOH aqueous electrolyte at room temperature hydroxide solution was then added dropwise to the suspension and under an argon atmosphere. Cyclic voltammograms (CV) until the pH was 9.5. The solution was stirred and the black were measured between 0.6 and 0.6 V (vs. SCE) and 0.6 to solid on the bottom of the eppendorfs was separated from the 0(vs. SCE) at different scan rates. blue solution by centrifugation. This composite material was washed several times with ethanol until the solvent was trans- parent. Subsequently, it was dried overnight in an oven at 60 C. 3. Results and discussion The composite of CNOs/4-DMAP/Ni(OH)2 with a mass ratio of 3.1. Preparation and characterization of Ni(OH)2 and NiO CNOs to Ni(OH)2 about 4 : 1 was thus obtained. The composites with nickel oxide (CNOs/4-DMAP/NiO) were obtained by calci- The synthesis of the precursors, Ni(OH)2 and NiO, in the pres- ff nation of CNOs/4-DMAP/Ni(OH)2 in an autoclave at 300 C ence of the di erent pyridine derivatives was performed. ff  during 2 hours. Three di erent modi ers were employed for Ni(OH)2 preparation, polyvinylpyrrolidone (PVP), (4-dimethylamino)pyridine 2.4. Apparatus (4-DMAP) and pyridinium p-toluenesulfonate polymer-bound (PPST) (Scheme 1). A control reaction was performed without The products and composite lms were imaged by secondary any modier (Scheme 1a (1)). electron SEM with the use of a INSPECT S50 scanning electron The results conrmed that this method offers a versatile microscope from FEI. The accelerating voltage of the electron means to fabricate Ni(OH)2 nanostructures with different beam was 15 keV and the working distance was 10 mm. morphologies (Fig. 1). The formation process of Ni(OH)2 occurs Transmission electron microscopy analyses were performed by the following steps:38 with a FEI instrument operated at 200 kV. The materials were 2+ 2+ sonicated in ethanol for 30 min and deposited on copper grids. Ni +NH3H2O / [Ni(NH3)y] + yH2O (1) Powder X-ray diffraction (XRD) (X'Pert Pro, Panalytica) were 2+ 2+ obtained using a Cu Ka source at a scanning speed of 0.0067 [Ni(NH3)y] / Ni + yNH3 (2) s 1 over a 2 Theta range of 10–90. Thermogravimetric experi- ments were performed using an SDT 2960 simultaneous TGA- DTG-DTA (TA Instruments company). The spectra were Published on 15 October 2013. Downloaded by The University of Texas at El Paso (UTEP) 05/05/2015 21:37:28. collected at 10 C min 1 in an air atmosphere (100 mL min 1). The Fourier Transform Infrared (FTIR) spectra were recorded between 4000 and 100 cm 1 with a Nicolet 6700 Thermo

Scientic spectrometer at room temperature and a N2 atmosphere. The spectra were collected with a resolution of 4 cm 1. All the spectra were corrected using conventional soware in order to cancel the variation of the analyzed thickness with the wavelength. The room-temperature Raman spectra at wave- lengths between 100 and 3500 cm 1 were investigated using a Renishaw spectrometer. To avoid sample overheating, the power of the laser beam was reduced to about 3 mW. The positions of the Raman peaks were calibrated using a Si thin lm as an external standard. The spectral resolution of the Raman spectra was 2 cm 1. The electrochemical measurements were obtained using a three-electrode conguration and a computer-controlled Auto- lab modular electrochemical system (Eco Chemie Ultecht, The Netherlands), using GPES soware (Eco Chemie Ultecht). The working electrode was a glassy carbon (GC) disk electrode

(Bioanalytical System Inc.) with a disk diameter of 2 mm. Prior Scheme 1 Synthesis of (a) Ni(OH)2, (b) CNO/4-DMAP/Ni(OH)2 and CNO/4- to the measurements the glassy carbon electrode was polished DMAP/NiO.

This journal is ª The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 25891–25901 | 25893 View Article Online RSC Advances Paper

annealing temperature was low enough to keep the nickel

hydroxide nanostructures intact. Ni(OH)2 prepared in the presence of polymers or 4-DMAP exhibited a diﬀerent morphology than that of the products aer the calcination process. SEM studies showed diﬀerent morphological characteristics and spectroscopic measurements (FT-IR and Raman) revealed that the nickel hydroxide (Fig. 2, panel 1a and 2a) and oxide materials (Fig. 2, panel 1b and 2b), exhibited the same structural characteristics. The FT-IR spectrum was used to determine the nature of the Ni samples before and aer calcination. The

FT-IR spectra of Ni(OH)2 are presented in Fig. 2 (panel 1a) and collected in Table 1. The broad peak in the region between 3000 and 3800 cm 1 was ascribed to the stretch vibration of O–H groups in the nickel hydroxide and oxide lattices, indicating the presence of lattice water.39 The band observed at 1617 cm 1 also indicated the presence of water. The NiO sample prepared at 300 C showed some water in the lattice as well. It was already

Fig. 1 SEM images of the Au foil covered with (a–d) Ni(OH)2 and (e–h) NiO obtained (a and e) without modiﬁer, in the presence of: (b and f) PVP, (c and g) 4- Published on 15 October 2013. Downloaded by The University of Texas at El Paso (UTEP) 05/05/2015 21:37:28. DMAP, and (d and h) PPST.

2+ Ni + 2OH / Ni(OH)2 (3)

NiO was also obtained by annealing the as-prepared Ni(OH)2 at 300 C for 2 h, according to eqn (4):

endothermic NiðOHÞ2 ! NiO þ H2O (4)

Fig. 1 presents the SEM images of pure Ni(OH)2 (Fig. 1a–d)

and NiO (Fig. 1e–h) structures. The morphology of Ni(OH)2 and NiO consists of particles of varied sizes and shapes, as a result of the addition of the modiers to the solution during the synthesis. The most noticeable diﬀerence in morphology was

observed for Ni(OH)2 obtained in the presence of PVP. This material showed a non-uniform structure with a ower-like Fig. 2 Spectroscopic characteristics of (a) Ni(OH)2 and (b) NiO obtained in the structure and grain particles. The nanostructures of the as- presence of 4-DMAP: (1) FTI-IR spectra in the range 4000–500 cm1. Spectra  prepared Ni(OH)2 were very similar to those of the NiO a er recorded at room temperature in a N2 atmosphere. (2) Room temperature Raman calcinating at 300 C, as shown in Fig. 1a and e. It seems that the spectra recorded for 514 nm excitation.

25894 | RSC Adv., 2013, 3, 25891–25901 This journal is ª The Royal Society of Chemistry 2013 View Article Online Paper RSC Advances

39–45 Table 1 Band frequencies and tentative assignment of the Raman and FTIR bands of Ni(OH)2 and NiO

FTIR (cm-1) Raman (cm-1)

a b a b Ni(OH)2 NiO Ni(OH)2 NiO Tentative assignment

3600 Vibrational stretching of hydroxyl group in the nickel hydroxide lattice 3500 Hydroxyl group of the intercalated and adsorbed water 2930 C–H stretching of aliphatic 1617 1617 Water angular deformation 1488 1298 1327 1280 1083 Nitrate anion 1045 1018 1040 1040 983 900 Ni–O vibrational stretching 700 Ni–O vibrational stretching 615 622 Ni–N vibrational stretching 520 Ni–O vibrational stretching 451 496 Ni–O vibrational stretching a Prepared in the presence of 4-DMAP. b Annealed at 300 C in an air atmosphere, 2 h.

reported that complete calcination occurs at 500 C, and pure 3.2. Preparation and characterization of the CNO/4-DMAP/ 40 NiO is obtained. Since Liu et al. reported that NiO calcinated at Ni(OH)2 and CNO/4-DMAP/NiO composites 300 C has a higher specic capacitance than the one calcinated The procedure for the modication of the CNOs surface with at 500 C, we used 300 C for our samples. Some of the bands nickel hydroxide and oxide is presented in Scheme 1b. To assigned to the calcinated NiO are blue shied compared to introduce nickel particles on the CNO surfaces, (4-dimethyla- those of Ni(OH) in bulk form due to a nanosize eﬀect.41 All of 2 mino)pyridine (4-DMAP) was used as a modier. 4-DMAP the Ni compounds have bands near the region of 1330–1300 and inuenced the properties of the nickel hydroxide nano- – 1 1080 980 cm , in which vibrations due to coordinated nitrate structures and simultaneously led to a pronounced increase of groups are known to occur.42 On the basis of these observations, the solubility of the carbon nanostructures in the protic the band at about 620 cm 1 has been assigned to a Ni–N solvents. stretching vibration of the coordinated nitrate group. The room temperature Raman spectrum of the mesoporous compounds showed bands at 520 and 451 cm 1, which can be attributed to the stretching (Ni–OH) vibration (Fig. 2, panel 2a), and at 496 cm 1, which is assigned to the stretch vibration of Ni–O (Fig. 2, panel 2b). The broad peak at 496 cm 1 is due to the

Published on 15 October 2013. Downloaded by The University of Texas at El Paso (UTEP) 05/05/2015 21:37:28. Ni–O stretching mode (Fig. 2, panel 2b),43 which can be iden-  44 ti ed as the A2u(T) lattice vibrations of nickel oxide. Mean- while, there are two other peaks (700 and 900 cm 1) that arise

from the mesoporous Ni(OH)2 structures, which contain nanoparticles but not single crystalline material.45 The electrochemical applications of materials depend on the degree of particle agglomeration in the lms, particle sizes, and other structural properties and defects on the surface of the nanostructures.46 High surface areas are required for high performance supercapacitors. Since it was already observed that

the crystal structure and morphology of Ni(OH)2 or NiO nanoparticles has a signicant inuence on its electrochemical properties,47 CV studies were performed for nickel oxides obtained in the presence of the diﬀerent modiers. The electrochemical characterizations in 1 M KOH electrolyte are shown in Fig. 3. The values of the capacitance for NiO varied between 58.9 and 105.6 F g 1 at 5 mV s 1, depending on the modier. The highest capacitance current was obtained for nickel oxides

calcinated from Ni(OH)2 prepared in the presence of 4-DMAP Fig. 3 Cyclic voltammograms of a GC electrode in 1 M KOH covered with NiO (Fig. 3). Thus this synthetic procedure was applied for the prepared without modiﬁer ( ), and in the presence of ( ) PVP, ( )4- 1 preparation of the CNO/4-DMAP/Ni(OH)2 composite. DMAP or ( ) PPST at (a) 5, (b) 20, and (c) 50 mV s scan rates.

This journal is ª The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 25891–25901 | 25895 View Article Online RSC Advances Paper

High-Resolution Transmission Electron Microscopy (HR- TEM) of the pure components of the composite: nickel hydroxide and oxide (Fig. 4a and b), was performed to determine the presence of inorganic structures in the composite. The

HR-TEM images of Ni(OH)2 clearly show (Fig. 4a) that the typical size of the particles is about 2–3 nm (structure 2, Fig. 4a). Fig. 4a

showed also that Ni(OH)2 reveals wrinkled surfaces (structure 1,

Fig. 4a). Ni(OH)2 exhibited similar ower shaped features with long needle-like type structures, which formed thin sheets. The unique sheet morphology with embedded mesoporous structures provides an ideal platform for fast ion transport, and has readily accessible surfaces for redox reactions.15 The electron- diﬀraction pattern with a diﬀused halo ring indicates that these nanospheres (not shown) with irregular structures have amorphous structures. Recently it was reported that amorphous Fig. 5 Room temperature Raman spectra of (a) CNO and CNO composites nickel hydroxide nanospheres exhibit high capacitance.48  obtained in the presence of 4-DMAP including (b) Ni(OH)2 and (c) NiO, recorded Ni(OH)2 a er calcination has more granular structures (struc- for 514 nm excitation. ture 3, Fig. 4b). The HR-TEM observation of the CNO composites (Fig. 4c–e) showed both structures: inorganic and carbon- based in the composite matrix. As shown in Fig. 4c, the typical structures are also visible in the CNO/4-DMAP/NiO composite

Ni(OH)2 amorphous nanospheres (structure 1) and CNO parti- (structures 3 and 4, Fig. 4e).

cles (structure 4) are present in the composite. Both types of The XRD pattern of the Ni(OH)2 and NiO products are presented in Fig. 4f. The large peak at 35.16 (003) reected the 49 typical turbostratic feature of a-Ni(OH)2 (Fig. 4f, pattern 1). All the diﬀraction peaks in Fig. 4f, pattern 1, are well indexed to the

pure hexagonal a-Ni(OH)2 phase (JCPDS card no. 03-0177): (003), (006), (012) and (110).49 The typical wide-angle XRD pattern of the NiO sample obtained aer calcination of a-  Ni(OH)2 is shown in Fig. 4e, pattern 2. Five well-de ned peaks, corresponding to the (111), (200), (220), (311) and (222) reec- tions were observed. They can be indexed to the cubic NiO crystalline phase structure (JCPDS card no. 04-0835).50

Fig. 5 shows the Raman spectra of the CNO/4-DMAP/Ni(OH)2 and CNO/4-DMAP/NiO composites and non-modied CNO for comparison. For all materials the strongest features are observed at about 1330 (D band) and 1580 cm 1 (G band). Published on 15 October 2013. Downloaded by The University of Texas at El Paso (UTEP) 05/05/2015 21:37:28. Moreover, in the spectral region from 300 cm 1 to 1200 cm 1 an additional two bands can also be observed, and according to the assignment by Dietz et al., these correspond to one-phonon (1P) LO modes (at about 540 cm 1) and two-phonon (2P) 2LO modes (1090 cm 1) (Fig. 5b and c).51 The phonon related part of the Raman spectra (1P and 2P bands) in the composite powders (Fig. 5b and c) is due to the presence of defects or surface eﬀects. Diﬀerential-thermogravimetric analyses (TGA-DTG-DTA) were performed to probe the thermal stability of pristine CNOs and their nickel composites in an air atmosphere. The temperature necessary for the removal of the porous carbon formed during the low-temperature annealing in an air atmosphere is lower than the decomposition temperature of the pristine carbon nanostructures. Fig. 6 shows the TGA-DTG-DTA scans under an air atmosphere at 10 C min 1 for the CNO composites (Fig. 6, panels (1) i (2), curves d and e), and the – reference materials: CNOs, Ni(OH)2 and NiO (curves a c, respectively). The onset oxidation and end temperatures are Fig. 4 HR-TEM images of (a) Ni(OH)2, (b) NiO, (c and d) CNO/4-DMAP/Ni(OH)2

and (e) CNO/4-DMAP/NiO. (f) XRD pattern of the (1) Ni(OH)2 and (2) NiO listed in Table 2, representing the initial weight loss, the products. maximum weight loss and the nal weight in the TGA-DTG-DTA

25896 | RSC Adv., 2013, 3, 25891–25901 This journal is ª The Royal Society of Chemistry 2013 View Article Online Paper RSC Advances

graphs, respectively. The highest inection temperature was Ni(OH)2 in the CNO composites can be estimated from the ¼ observed for the pristine CNOs (It 610 C). There is no weight weight loss of the TGA curves. At temperatures of 260 and loss or very small loss at temperatures between 200 and 400 C, 400 C, the weight percentages are 33 and 15%, which means

showing the small content of amorphous carbon in the pristine the mass ratio of Ni(OH)2 or NiO is about 4 : 1. CNO samples. Very sharp transitions are observed at 261 and 408 C that correspond to the conversion of Ni(OH)2 to NiO and 52 H2O (Fig. 6, panel 2, curve b). 3.3. Morphology characterization and electrochemical The same properties were also observed for the CNO/4- studies DMAP/Ni(OH)2 composite (Fig. 6, panel 2, curve d). According to The capacitance values are strictly correlated with the nature the TGA studies, we chose 300 C as the annealing temperature and surface of the electrode–electrolyte interface. Generally, the for the production of NiO materials. The percentage of NiO and larger the specic surface the higher the charge accumula- 3,5 tion. The CNO/4-DMAP/Ni(OH)2 and CNO/4-DAMP/NiO composites were obtained via synthesis of the carbon nanostructures in the presence of 4-DMAP as modier. The morphology and roughness of the synthesized CNO composite layers were studied by scanning electron microscopy. The results of the SEM studies are shown in Fig. 7. The morphology of the lms formed by using modied CNO with Ni particles (Fig. 7d and e), does not diﬀer signicantly from that of lms

formed by non-functionalized nanostructures: Ni(OH)2/NiO (Fig. 7b and c) or CNO (Fig. 7a). Cyclic voltammetry was used to determine the electro-

chemical properties of NiO/Ni(OH)2 and the CNO composites including such particles. As was previously observed, the capacitive performance of the carbon electrodes depends on their preparation protocol. Our previous preparation of solid lms including CNO structures was based on the drop coating method using pure solvents. A drop of solution containing the dispersed carbon nanostructures were deposited on the electrode surface. Aer solvent evaporation, the electrode covered with the thin lm was transferred to a solution containing the supporting electrolyte. These electrodes showed good mechanical and electrochemical stability, and some aggregation of the carbon-based materials was observed. The specic capacitance for the GC electrode covered with the pristine CNOs was equal to 7 F g 1.53 For the preparation of the lms a Published on 15 October 2013. Downloaded by The University of Texas at El Paso (UTEP) 05/05/2015 21:37:28. conductive carbon paste was used to solubilize the non-modi- ed and modied CNOs for further modication of the GC surface electrodes. The volume ratio of carbon paste to ethanol used was 1 : 6. A drop of 10 mL ethanol solution containing the dispersed CNOs was deposited on the electrode surface. Aer solvent evaporation, the electrode covered with the thin lm

Fig. 6 (1) TGA and (2) DTG curves for (a) CNO (black line), (b) Ni(OH) (blue line), was transferred to a solution with the supporting electrolyte. All 2  (c) NiO (gray line), (d) CNO/4-DMAP/Ni(OH)2 (green line), and (e) CNO/4-DMAP/ voltammograms exhibit pseudorectangular pro les that are NiO (red line) in an air atmosphere at 10 Cmin1. typical for double-layer capacitors (Fig. 8). These results

Table 2 The TGA-DTG-DTA results of CNOs and their Ni(OH)2 and NiO composites

Onset Inection End Total Sample temperature (C) temperature (C) temperature (C) weight loss (%)

CNO 490 610 640 98 a Ni(OH)2 261 200 490 33 NiOa,b 408 280 490 15 a CNO/Ni(OH)2 261, 408 180 690 52 CNO/NiOa,b 408 240 680 34 a Prepared in the presence of 4-DMAP. b Annealed at 300 C in an air atmosphere, 2 h.

This journal is ª The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 25891–25901 | 25897 View Article Online RSC Advances Paper

Table 3 Speciﬁc capacitance of CNO-based materials obtained from the voltammetric studiesa

Specic capacitance (F g 1)f at diﬀerent scan rates (mV s 1)

Sample 5 10152030

CNO 30.6 37.9 38.5 39.1 39.1 36.5d 43.3d 44.1d 44.7d 44.8d b Ni(OH)2 548.4 472.6 482.3 423.0 387.0 NiOb,c 83.6 82.3 72.4 78.2 77.2 b e e e e e CNO/Ni(OH)2 1225.2 904.3 871.3 773.0 727.4 CNO/NiOb,c 290.6e 218.2e 222.5e 230.5e 272.5e a Capacitance per gram of the composite. b Prepared in solution with 4- c DMAP. Ni(OH)2 annealed at 300 C in an air atmosphere, 2 h. d Capacitance in 0.1 M NaCl. e Capacitance per gram of CNOs into f Ni(OH)2 or NiO. 1 M KOH.

CNOs caused by van der Waals interactions, consequently maintaining an electrochemically active surface area and a suitable porous structure. Adding conductive carbon paste for CNO lm preparation was more eﬀective than using pure solutions to enhance the performance of EDLC, with approxi- mately 20 times larger specic capacitances, about 45 F g 1

(Table 3). The specic capacitance, Cs, was calculated based on the mass of the CNOs on the electrode surface, m, within the potential range, DE, (Fig. 8) according to the following equation: ð

Fig. 7 SEM images of the Au foil covered with (a) CNOs, (b) Ni(OH)2, (c) NiO, (d) ic dt fi CNO/4-DMAP/Ni(OH)2, and (e) CNO/4-DMAP/NiO composites. The lm was C ¼ s DEm (5) prepared as follows: 2 mg of active material (CNOs, Ni(OH)2, NiO, CNO/4-DMAP/ Ni(OH) or CNO/4-DMAP/NiO) was dissolved in a solution containing conductive 2  1 carbon paint and ethanol (volume ratio of 1 : 6). The CNO lms show capacity ranges between 30 and 39 F g in 1 M KOH (Fig. 8a (3)) and 36 and 45 in 0.1 M NaCl (Fig. 8b (2) and Table 3). indicate that the charge–discharge responses of the electric The CNO/4-DMAP/Ni(OH)2 and CNO/4-DMAP/NiO  double layer are highly reversible and kinetically facile.54 The composite lms exhibited good mechanical and electro- capacitive current characteristics for pristine CNOs prepared in chemical stability under cyclic voltammetric conditions within the potential range from 0.60 to +0.60 V versus SCE. Fig. 9 Published on 15 October 2013. Downloaded by The University of Texas at El Paso (UTEP) 05/05/2015 21:37:28. the presence of carbon paste was higher than those obtained by the drop coating method. Since the specic capacitance shows the CV curves of the GC electrodes covered with inor-  ff depends on the degree of particle agglomeration in the lm, the ganic/carbon-based lm electrodes at di erent scan rates in 1 M conductive carbon paste allowed the preparation of good KOH aqueous solution. A nickel oxide/hydroxide capacitor in dispersions of CNOs in ethanol solution. The presence of alkaline solution relies on charge storage in the electric double – conductive carbon paste is able to prevent the aggregation of layer at the electrode electrolyte interface and charge storage in the host material through redox reactions on the surface and hydroxyl ion diffusion in the host material.55 The redox peaks reveal the faradaic pseudocapacitive nature due to the Ni2+/Ni3+

couple at the surface of Ni(OH)2 (Fig. 9a) and NiO (Fig. 9b), according to the following equations:

Ni(OH)2 +OH 4 NiOOH + H2O+e (6)

NiO + OH 4 NiOOH + e (7)

The oxidation peak at 0.50 V (vs. SCE) is due to the conversion of NiO to NiOOH, whereas the reduction at 0.25 (vs. SCE) is Fig. 8 (a) Cyclic voltammograms of a GC electrode covered with (1) carbon paste, (2 and 3) CNOs; CVs (1) and (2) in 0.1 M NaCl, and (3) in 1 M KOH; scan rates due to the reverse reaction. A nickel hydroxide surface layer a 56 5mVs 1; (b) cyclic voltammograms of a GC electrode covered with CNOs in 0.1 M few angstroms thick is formed. The CNO/4-DMAP/Ni(OH)2 NaCl with the diﬀerent scan rates: 5, 10, 15, 20 and 30 mV s 1. composite shows similar electrochemical performance as

25898 | RSC Adv., 2013, 3, 25891–25901 This journal is ª The Royal Society of Chemistry 2013 View Article Online Paper RSC Advances

values of Cs for NiO or Ni(OH)2 varied between 77.2 and 548.4 F g 1. The role of the carbon nanostructures with high specic surface area is to supply more active sites for the redox reaction 57  of Ni(OH)2 or NiO. A speci c capacitance between 218.2 and 1225.2 F g 1 was obtained based on the composites mass,

CNOs/4-DMAP/NiO or CNOs/4-DMAP/Ni(OH)2, respectively. According to an earlier investigation,58 the composite's struc-

ture is important for Ni(OH)2/NiO to function as a fast charging and discharging supercapacitor and is responsible for the high pseudocapacitance value. Moreover, the presence of CNOs in the lm results in an increase of the material's conductivity. The

electrochemical performance of the CNO/4-DMAP/Ni(OH)2 or CNO/4-DMAP/NiO composites is largely aﬀected by the 59 morphology and distribution of the Ni(OH)2/NiO phase. In conclusion, CNOs improve the composite lm properties, making them more active for faradaic reactions, thus conferring larger specic capacitances and better stabilities than for pris-

tine Ni(OH)2 or NiO.

4. Conclusion

We have shown that functionalization of carbon nano-onions

with inorganic materials, such as Ni(OH)2 or NiO, can be easily achieved. The structure and physico-chemical characterization of the composites was investigated by TEM, SEM, FT-IR, Raman

and TGA-DTG-DTA. The CNO/4-DMAP/Ni(OH)2 and CNO/4- DMAP/NiO lms showed relatively porous structures on the electrode surface and exhibit typical pseudocapacitive behav- iour, as well as good mechanical and electrochemical stability Fig. 9 Cyclic voltammograms of a GC electrode in 1 M KOH covered with: (a) over a wide potential window, from +0.6 to 0.6 V (vs. SCE). The  Ni(OH)2/4-DMAP, (b) NiO/4-DMAP, (c) CNOs/4-DMAP/Ni(OH)2, (d) CNOs/ speci c electrochemical capacitance of the CNO/4-DMAP/ ﬀ 4-DMAP/NiO and (e) CNOs with the di erent scan rates: 5, 10, 15, 20 and Ni(OH)2 and CNO/4-DMAP/NiO composite lms was signi- 30 mV s1; (f) dependence of the capacitive current with the sweep rate for: (C) cantly larger than for pure NiO or Ni(OH)2. The highest specic CNOs, (,) NiO/4-DMAP, (-) CNOs/4-DMAP/NiO, (>) Ni(OH) /4-DMAP and (A) 2 capacitance of 1225.2 F g 1 was observed for the CNO/4-DMAP/ CNOs/4-DMAP/Ni(OH)2 at +100 mV for CNOs and at O1 peak.  Ni(OH)2 composite lms. The simple synthesis of these materials, their cation exchange properties, and their facile electro-

Published on 15 October 2013. Downloaded by The University of Texas at El Paso (UTEP) 05/05/2015 21:37:28. chemistry make them attractive for applications as electrodes Ni(OH)2 (Fig. 9c), but the separation between oxidation and reduction peaks is decreased by 40 mV. for supercapacitors.

The capacitive current, ic, is given by the following equation: Acknowledgements ic ¼ CSvm (8) The authors thank A. Cyganiuk (Nicolaus Copernicus Univer-

where Cs is the specic capacitance, m is the mass of the sity, Torun, Poland) for XRD and TGA-DTG-DTA; J. Breczko material deposited on the electrode surface, and v is the (University of Bialystok) for FT-IR and Raman measurements. potential sweep rate. The values of the specic capacitances We gratefully acknowledge the nancial support of the NCN, calculated from the dependence of the current with the scan Poland, grant: #2011/01/B/ST5/06051 to M. E. P.-B. L. E. thanks rate for the diﬀerent types of lms (eqn (8)), are collected in the Robert A. Welch Foundation for an endowed chair, grant Table 3. The values of the specic capacitances were calculated #AH-0033 and the US NSF, grants: CHE-1110967 and CHE- from the linear relationship of I–v plots (Fig. 9f). 1124075. SEM and TEM, IR and Raman instruments were fun-

The capacitive behavior of the CNOs/4-DMAP/Ni(OH)2 and ded by the European Funds for Regional Development and the CNOs/4-DMAP/NiO composites are mainly attributed to the National Funds of Ministry of Science and Higher Education, as pseudocapacitive performance of nickel oxide or hydroxide, but part of the Operational Programme Development of Eastern the carbon nano-onions signicantly increased the roughness Poland 2007-2013, project: POPW.01.03.00-20-034/09-00. and specic surface areas of the nanocomposite electrodes. The capacitances of CNO lms are very low when compared to those Notes and references of the composites. A specic capacitance between 30.6 and 39.1 Fg 1 was obtained based on the CNOs mass. Much higher 1R.Kotz¨ and M. Carlen, Electrochim. Acta, 2000, 45, 2483.

This journal is ª The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 25891–25901 | 25899 View Article Online RSC Advances Paper

2 B. B. Owens and T. Osaka, J. Power Sources, 1997, 68, 173; Z.-H. Liang, L. Li and L. Zhang, J. Solid State Chem., 2007, G. Wang, L. Zhang and J. Zhang, Chem. Soc. Rev., 2012, 41, 180, 2095; Y.-Z. Zheng, H.-Y. Ding and M.-L. Zhang, Mater. 797; E. Frackowiak and F. Beguin, Carbon, 2001, 39, 937; Res. Bull., 2009, 44, 403. E. Frackowiak, Phys. Chem. Chem. Phys., 2007, 9, 1774. 23 A. M. Soleimanpour and A. H. Jayatissa, Mater. Sci. Eng. C, 3 S. Bose, T. Kuila, A. K. Mishra, R. Rajasekar, N. H. Kim and 2012, 32, 2230. J. H. Lee, J. Mater. Chem., 2012, 22, 767. 24 M.-S. Wu, Y.-A. Huang, C.-H. Yang and J.-J. Jow, Int. J. 4 J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. Simon and Hydrogen Energy, 2007, 32, 4153. P. L. Taberna, Science, 2006, 313, 1760; A. G. Pandolfo and 25 J. D. Desai, S.-K. Min, K.-D. Jung and O.-S. Joo, Appl. Surf. Sci., A. F. Hollenkamp, J. Power Sources, 2006, 157, 11; 2006, 253, 1781. M. D. Levi, G. Salitra, N. Levy and D. Aurbach, Nat. Mater., 26 C. C. Hu, W. C. Chen and K. H. Chang, J. Electrochem. Soc., 2009, 8, 872. 2004, 151, A281; H. Kuan-Xin, W. Quan-Fu, Z. Xiao-Gang 5 S. Park, H. Ch. Kim and T. D. Chung, Analyst, 2012, 137, and W. Xin-Lei, J. Electrochem. Soc., 2006, 153, A1568–A1574. 3891. 27 X. Huang, X. Qi, F. Boey and H. Zhang, Chem. Soc. Rev., 2012, 6 F. Fusalba, H. A. Ho, L. Breau and D. Belanger, Chem. Mater., 41, 666; Ch.-W. Huang, Ch.-T. Hsieh, P.-L. Kuo and H. Teng, 2000, 12, 2581; K. K. Liu, Z. L. Hu, R. Xue, J. R. Zhang and J. Mater. Chem., 2012, 22, 7314; M. Wu, G. A. Snook, V. Gupta, J. J. Zhu, J. Power Sources, 2008, 179, 858. M. Shaﬀer, D. J. Fray and G. Z. Chen, J. Mater. Chem., 2005, 7 X. Dong, W. Shen, J. Gu, L. Xiong, Y. Zhu, H. Li and J. Shi, J. 15, 2297; R. B. Moghaddam and P. G. Pickup, Phys. Chem. Phys. Chem. B, 2006, 110, 6015; K. R. Prasad, K. Koga and Chem. Phys., 2010, 12, 4733; N. A. Kumar, H. J. Choi, N. Miura, Chem. Mater., 2004, 16, 1845; T. Y. Wie, A. Bund, J.-B. Baek and Y. T. Jeong, J. Mater. Chem., 2012, C. H. Chen, H. C. Chien, S. Y. Lu and C. C. Hu, Adv. 22, 12268; J. H. Park, O. O. Park, K. H. Shin, Ch. S. Jin and Mater., 2010, 21, 347. J. H. Kim, Electrochem. Solid-State Lett., 2002, 5, H7; S. Arai, 8 M. Mastragostinoa, C. Arbizzani and F. Soavi, Solid State M. Endo, T. Sato and A. Koide, Electrochem. Solid-State Ionics, 2002, 148, 493. Lett., 2006, 9, C131; S. Arai, T. Saito and M. Endo, J. 9 J. Chang, J. Sun, C. Xu, H. Xu and L. Gao, Nanoscale, 2012, 4, Electrochem. Soc., 2007, 154, D530; T. Okumura, T. Sugiyo, 6786. T. Inoue, M. Ikegami and T. Miyasaka, J. Electrochem. Soc., 10 Z. A. Hu, Y. L. Xie, Y. X. Wang, H. Y. Wu, Y. Y. Yang and 2013, 160, H155; Z. Tai, J. Lang, X. Yan and Q. Xue, J. Z. Y. Zhang, Electrochim. Acta, 2009, 54, 2737. Electrochem. Soc., 2012, 159, A485. 11 M.-W. Xu, D.-D. Zhao, S.-J. Bao and H.-L. Li, J. Solid State 28 R. P. Deo, N. S. Lewrence and J. Wang, Analyst, 2004, 129, Electrochem., 2007, 11, 1101; B. Wang, J. Park, C. Wang, 1076; C.-T. Hsieh, Y.-W. Chou, W.-Y. Chen and J.-Y. Lin, H. Ahn and G. Wang, Electrochim. Acta, 2010, 55, 6812. Int. J. Hydrogen Energy, 2007, 32, 3457; A. S. Adekunle and 12 J. Yan, T. Wei, W. Qiao, B. Shao, Q. Zhao, L. Zhang and K. I. Ozoemena, Electrochim. Acta, 2008, 53, 5774; Z. Fan, Electrochim. Acta, 2010, 55, 6973. A. Czerwinski, M. Dmochowska, M. Grden, M. Kopczyk, 13 M.-S. Wu, C.-Y. Huang and K.-H. Lin, J. Power Sources, 2009, G. Wojcik, G. Mlynarek, J. Kolata and J. M. Skowronski, J. 186, 557. Power Sources, 1999, 77, 28; K. T. Roro, N. Tile and 14 Y. Wang and G. Z. Cao, Key Eng. Mater., 2007, 336–338, 2134. A. Forbes, Appl. Surf. Sci., 2012, 258, 7174; Ch. Xu, J. Sun 15 X. Sun, G. Wang, J.-Y. Hwang and J. Lian, J. Mater. Chem., and L. Gao, J. Power Sources, 2011, 196, 5138; G.-H. Yuan,

Published on 15 October 2013. Downloaded by The University of Texas at El Paso (UTEP) 05/05/2015 21:37:28. 2011, 21, 16581. Z.-H. Jiang, A. Aramata and Z. Gao, Carbon, 2005, 43, 2913. 16 L. Dong, Y. Chu and W. Sun, Chem.–Eur. J., 2008, 14, 5064. 29 V. L. Kuznetsov, A. L. Chuvilin, Y. V. Butenko, I. Y. Malkov and 17 K. C. Liu and M. A. Anderson, J. Electrochem. Soc., 1996, 143, V. M. Titov, Chem. Phys. Lett., 1994, 222,343;J.Qian, 124; K. W. Nam and K. B. Kim, J. Electrochem. Soc., 2002, 149, C. Pantea, J. Huang, T. W. Zerda and Y. Zhao, Carbon, 2004, A346. 42, 2691; Q. Zou, Y. G. Li, B. Lv, M. Z. Wang, L. H. Zou and 18 C. Z. Yuan, B. Gao and X. G. Zhang, J. Power Sources, 2007, Y. C. Zhao, Inorg. Chem.,2010,46, 127; A. Palkar, F. Melin, 173, 606; C. Z. Yuan, B. Gao, L. H. Su and X. G. Zhang, C. M. Cardona, B. Elliott, A. K. Naskar, D. D. Edie, Solid State Ionics, 2008, 178, 1859; F. Jiao, A. H. Hill, A. Kumbhar and L. Echegoyen, Chem.–Asian J., 2007, 2, 625. A. Harrison, A. Berko, A. V. Chadwick and P. G. Bruce, J. 30 S. Sek, J. Breczko, M. E. Plonska-Brzezinska, A. Z. Wilczewska Am. Chem. Soc., 2008, 130, 5262. and L. Echegoyen, ChemPhysChem, 2013, 14, 96. 19 H.-Y. Wu and H.-W. Wang, Int. J. Electrochem. Sci., 2012, 7, 31 M.-S. Wang, D. Goldberg and Y. Bondo, ACS Nano, 2010, 4, 4405. 4396. 20 M. S. Wu, M. J. Wang and J. J. Jow, J. Power Sources, 2010, 32 S. Tomita, M. Fujii and S. Hayashi, Phys. Rev. B: Condens. 195, 3950; V. Ganesh, S. Pitchumani and Matter Mater. Phys., 2002, 66, 245424; V. L. Kuznetzov, V. Lakshminarayanan, J. Power Sources, 2006, 158, 1523; S. I. Moseenkov, K. V. Elumeeva, T. V. Larina, H. L. Wang, H. S. Casalongue, Y. Y. Liang and H. J. Dai, J. V. F. Anufrienko, A. I. Romanenko, O. B. Anikeeva and Am. Chem. Soc., 2010, 132, 7472; K. Liang, X. Tang and E. N. Tkachev, Phys. Status Solidi B, 2011, 248, 2572; W. Hu, J. Mater. Chem., 2012, 22, 11062. T. Cabioc'h, S. Camelio, L. Henrard and Ph. Lambin, Eur. 21 G.-W. Yang, C.-L. Xu and H.-L. Li, Chem. Commun., 2008, 6537. Phys. J. B, 2000, 18, 535. 22 M. Salavati-Niasari, H. Seyghalkar, O. Amiri and F. Davar, J. 33 M. E. Plonska-Brzezinska, D. M. Brus, J. Breczko and Cluster Sci., 2013, 24, 365; L.-X. Yang, Y.-J. Zhu, H. Tong, L. Echegoyen, Chem.–Eur. J., 2013, 19, 5019; J. Breczko,

25900 | RSC Adv., 2013, 3, 25891–25901 This journal is ª The Royal Society of Chemistry 2013 View Article Online Paper RSC Advances

M. E. Plonska-Brzezinska and L. Echegoyen, Electrochim. 42 C. W. Fraxk and L. B. Rogers, Inorg. Chem., 1966, 5, 622. Acta, 2012, 72, 61; J. Luszczyn, M. E. Plonska-Brzezinska, 43 S. I. Cordoba-Torresi, A. Hugot-Le Goff and S. Joiret, J. A. Palkar, A. T. Dubis, A. Simionescu, D. T. Simionescu, Electrochem. Soc., 1991, 138, 1554. B. Kalska-Szostko, K. Winkler and L. Echegoyen, Chem.– 44 J. Yan, W. Sun, T. Wei, Q. Zhang, Z. Fan and F. Wei, J. Mater. Eur. J., 2010, 16, 4870. Chem., 2012, 22, 11494. 34 Y. Liu, R. L. Vander Wal and V. N. Khabashesku, Chem. 45 B. Li, M. Ai and Z. Xu, Chem. Commun., 2010, 46, 6267. Mater., 2007, 19, 778. 46 P. Simon and Y. Gogotsi, Philos. Trans. R. Soc., A, 2010, 368, 35 J. K. McDonough, A. I. Frolov, V. Presser, J. Niu, 3457. Ch. H. Miller, T. Ubieto, M. V. Federov and Y. Gogotsi, 47 M. C. Bernard, R. Cortes, M. Keddam, H. Takenouti, Carbon, 2012, 50, 3298; C. Portret, G. Yushin and P. Bernard and S. Senyarich, J. Power Sources, 1996, 63, Y. Gogotsi, Carbon, 2007, 45, 2511; G. Salitra, A. Soffer, 247; K. Watanabe and T. Kikuoka, J. Appl. Electrochem., L. Eliad, Y. Cohen and D. Aurback, J. Electrochem. Soc., 1995, 25, 219; D. E. Reisner, A. J. Salkind, P. R. Strutt and 2000, 147, 2486; C. Vix-Guterl, E. Frackowiak, K. Jurewicz, T. D. Xiao, J. Power Sources, 1997, 65, 231. M. Friebe, J. Parmantier and F. Beguin, Carbon, 2005, 43, 48 H. B. Li, M. H. Yu, F. X. Wang, P. Liu, Y. Liang, J. Xiao, 1293; L. Eliad, G. Salitra, A. Soffer and D. Aurbach, C. X. Wang, Y. X. Tong and G. W. Yang, Nat. Commun., Langmuir, 2005, 21, 3198; R. Lin, P. Huang, J. Segalini, 2013, 4, 1894. C. Largeot, P. L. Taberna, J. Chmiola, Y. Gogotsi and 49 B. C. Cornilsen, P. J. Karjala and P. L. Loyselle, J. Power P. Simon, Electrochim. Acta, 2009, 54, 7025; E. Raymundo- Sources, 1988, 22, 351; H. B. Liu, L. Xiang and Y. Jin, Cryst. Pinero, K. Kierczek, J. Machnikowski and F. Begiun, Growth Des., 2006, 6, 283; M. Salavati-Niasari, Carbon, 2006, 44, 2498; J. Chmiola, C. Largeot, H. Seyghalkar, O. Amiri and F. Davar, J. Cluster Sci., 2013, P.-L. Taberna, P. Simon and Y. Gogotsi, Angew. Chem., Int. 24, 365. Ed., 2008, 47, 3392; M. E. Plonska-Brzezinska, A. Palkar, 50 Y.-z. Zheng, H.-y. Ding and M.-l. Zhang, Mater. Res. Bull., K. Winkler and L. Echegoyen, Electrochem. Solid-State Lett., 2009, 44, 403; X.-M. Liu, X.-G. Zhang and S.-Y. Fu, Mater. 2010, 13, K35; M. E. Plonska-Brzezinska, J. Mazurczyk, Res. Bull., 2006, 41, 620; Ch. Yuan, J. Li, L. Hou, L. Yang, J. Breczko, B. Palys, A. Lapinski and L. Echegoyen, Chem.– L. Shen and X. Zhang, Electrochim. Acta, 2012, 78, 532. Eur. J., 2012, 18, 2600; E. Gradzka, K. Winkler, 51 R. E. Dietz, W. F. Brinkman, A. E. Meixner and M. Borowska, M. E. Plonska-Brzezinska and L. Echegoyen, H. J. Guggenheim, Phys. Rev. Lett., 1971, 27, 81. Electrochim. Acta, 2013, 96, 274; J. Breczko, K. Winkler, 52 W. Xing, F. Li, Z. F. Yan and G. Q. Lu, J. Power Sources, 2004, M. E. Plonska-Brzezinska, A. Villalta-Cerdas and 134, 324. L. Echegoyen, J. Mater. Chem., 2010, 20, 7761. 53 M. E. Plonska-Brzezinska, M. Lewandowski, M. Błaszyk, 36 Y.-G. Wang, L. Yu and Y.-Y. Xia, J. Electrochem. Soc., 2006, A. Molina-Ontoria, T. Lucinski´ and L. Echegoyen, 153, A743. ChemPhysChem, 2012, 13, 4134. 37 X. Liu, S. W. Or, Ch. J., Y. Lv, Ch. Feng and Y. Sun, Carbon, 54 S. R. S. Prabaharan, R. Vimala and Z. Zainal, J. Power Sources, 2013, 60, 215. 2006, 161, 730. 38 R. L. Penn and J. F. Baneld, Science, 1998, 281, 969; B. Liu, 55 R. Bernard, C. F. Randell and F. L. Tye, J. Appl. Electrochem., S. H. Yu, L. J. Li, F. Zhang, Q. Zhang, M. Yoshimura and 1980, 10, 109.

Published on 15 October 2013. Downloaded by The University of Texas at El Paso (UTEP) 05/05/2015 21:37:28. P. K. Shen, J. Phys. Chem. B, 2004, 108, 2788; B. J. Xi, 56 P. A. Nelson and J. R. Owen, J. Electrochem. Soc., 2003, 150, S. L. Xiong, D. C. Xu, J. F. Li, H. Y. Zhou, J. J. Y. Pan and A1313; K. W. Nam, E. S. Lee, J. H. Kim, Y. H. Lee and Y. T. Qian, Chem.–Eur. J., 2008, 14, 9786; Ch. Yuan, K. B. Kim, J. Electrochem. Soc., 2005, 152, A2123; X. Zhang, L. Su, B. Gao and L. Shen, J. Mater. Chem., 2009, I. Bouessay, A. Rougier and M. Tarascon, J. Electrochem. 19, 5772. Soc., 2004, 151, H145; K. W. Nam, K. H. Kim, E. S. Lee, 39 M. B. J. G. Freitas, J. Power Sources, 2001, 93, 163; W. S. Yoon, X. Q. Yang and K. B. Kim, J. Power Sources, M. B. J. G. Freitas, R. K. Silva Silva, D. M. A. Rozario and 2008, 182, 642. P. G. Manoel, J. Power Sources, 2007, 165, 916. 57 Y.-z. Zheng and M.-l. Zhang, ECS Trans., 2007, 2(8), 51. 40 X.-M. Liu, X.-G. Zhang and S.-Y. Fu, Mater. Res. Bull., 2006, 58 C. Yuan, X. Zhang, L. Su, B. Gao and L. Shen, J. Mater. Chem., 41, 620. 2009, 19, 5772. 41 A. J. Hunt, T. R. Steyer and D. R. Huﬀman, Surf. Sci., 1973, 36, 59 P. Lin, Q. She, B. Hong, X. Liu, Y. Shi, Z. Shi, M. Zheng and 454. Q. Dong, J. Electrochem. Soc., 2010, 157, A818.

This journal is ª The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 25891–25901 | 25901